Synthetic hydrocarbon fluids useful as lubricant compositions are well-known in the art. For example, one such synthetic hydrocarbon fluid is that obtained by the catalytic polymerization of .alpha.-olefins. In general, the synthetic lubricants provide lower friction and, hence, increase mechanical efficiency across the full spectrum of mechanical loads and do so over a wider range of operating conditions relative to mineral oil lubricants.
Synthetic lubricants made by polymerizing ethylene are especially desired because the raw materials are readily accessible and the process is overall more economically efficient. The production of polyethylene fluid is economically efficient because the production of polyethylene fluids is a single step process. In contrast, the production of poly .alpha.-olefin fluids, for example, requires ethylene to be first oligomerized to form the .alpha.-olefin.
The objective of industrial research on synthetic lubricants is, in general, to achieve a polymeric fluid that exhibits a useful viscosity over a wide range of temperature, i.e., has a good viscosity index (VI), while also exhibiting good lubricity, and a pour point equal to or better than mineral oil. One characteristic of the molecular structure of the polymeric fluids has been found to correlate very well with all of these desirable lubricant properties. This characteristic is the polymer's branching index, BI. BI is the ratio of methyl protons to total non-benzylic, aliphatic group protons in the polymer product and is easily determined from proton NMR spectra by calculating the ratio in percent of non-benzylic methyl hydrogens in the range of 0.5 to 1.05 ppm, to the total non-benzylic aliphatic hydrogens in the range of 0.5 to 2.1 ppm.
Generally, as the BI increases, the pour point of the polyethylene fluid, i.e., the temperature at which the composition changes from a liquid to a solid, decreases. This is a desirable effect as a lower pour point extends the application range of the polyethylene fluid. BI, however, has a negative effect on the viscosity index of a polyethylene oil; it is well-known in the art that the viscosity index of polyethylene fluids decreases as the branching index increases. This is an undesirable effect because a lower viscosity index indicates a poor viscosity-temperature performance. Thus, the challenge in synthesizing polyethylene fluids is to achieve an amount of branching sufficient to maintain the polyethylene in a liquid state such that the polyethylene fluid has a good viscosity index.
Polymerization of ethylene by transition metal catalysts, for example, usually leads to the formation of solid, linear polymers. As such, they are not suitable as soft materials or lubricants for most applications.
Recently, DuPont and the University of North Carolina, have developed novel Ni(II)- and Pd(II)-based catalysts which catalyze the polymerization of ethylene to form polyethylene liquids. These Nickel(II) and Palladium(II)-based catalysts contain chelating ligands which, as stated to, greatly reduce chain transfer termination rates and, thus, lead to the formation of high molecular weight polymers. In these systems, however, the degree of branching is only 20 to 150 branches per 1000 CH.sub.2 groups.
Other highly active Nickel(II) and Palladium(II)-based catalysts have been discovered by Penn State University and have been employed for the preparation of highly-branched polyethylene fluids having greater than 587 branches per 1000 CH.sub.2 groups. These polymers and their synthesis have been reported by J. S. Kim, J. H. Powlow, L. M. Wojcinski, S. Murtuza, S. Kacker, and A. Sen, "Novel Nickel(II) and Palladium(II)-Based Catalyst Systems for the Synthesis of Hyperbranched Polymers from Ethene," J. Am. Chem. Soc. 120, 1932, 1998. Such highly branched polymers, however, have a VI that typically is too low to be used as, for example, a lube basestock.
A report from Switzerland [Adv. Poly. Sci. 1974, 15, 1] describes the use of a TiCl.sub.4 /EtAlCl.sub.2 catalyst for the polymerization of ethylene in benzene to give branched polymers. The products obtained, however, were of relatively low branching and molecular weight. One detrimental side reaction associated with the use of benzene solvent is Friedel-Crafts alkylation of the solvent and thus, a large portion of the product is reported to contain aromatic rings.
Thus, there is a need in the art for polyethylene fluids having a molecular weight, a branching index, and a viscosity index such that they are suitable for use as synthetic lubricants and processes for the manufacture of such polyethylene fluids.